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MOTION OF A PARTICLE                       101
the block is exactly balanced by the reaction of the plane so that there are no forces to be overcome in order to move the block. Therefore we conclude that the block resists any attempt to start it into motion. In other words, the block offers resistance to a force which accelerates it. This resistance is the kinetic reaction.
In one respect kinetic reaction is similar to frictional and resisting forces, namely, it is not aggressive. The kinetic reaction of a body manifests itself only when its state of rest or of motion is interfered with. A body which is at rest or moving with a constant velocity does not display any kinetic reaction, but as soon as it is set in motion or its velocity is changed kinetic reaction appears; further the kinetic reaction of a body is greater the greater the acceleration imparted to it.
93. Generalization of the Law of Action and Reaction.  When the terms "action7' and "reaction" are used so as to mean kinetic reactions as well as forces and torques, then the law is directly applicable to problems of motion as well as to problems of equilibrium. It will be remembered that in Chapter III the law was split into two sections, of which the second section is not applicable to single particles. Therefore we need to consider here only the first section, which states:
To every linear action there is always an equal and opposite linear reaction, or the sum of all the linear actions to which a body or a part of a body is subject at any instant vanishes.
SAZ = 0.                                     (A,)
If we replace the term "linear action" by the terms "force" and "linear* kinetic reaction'7 the law may be put into the following form.
* The adjective "linear" is introduced in order to distinguish between the kinetic reaction which is related to forces and the kinetic reaction introduced at the beginning of Chapter IX, which is related to torques.